This invention relates to a device for lighting a discharge lamp in which a discharge bulb is a light source (hereinafter referred to as "a discharge lamp lighting device", when applicable), and more particularly to a discharge lamp lighting device with a lighting circuit whose output is stabilized.
Recently, research has been conducted on the application of a lamp whose light source is a discharge bulb which is high in luminous efficiency, excellent in color rendering property and long in service life to a head-light of an automobile. A lamp of this type needs a high voltage to cause electric discharge in the discharge bulb. Hence, it is necessary to equip the lamp with a lighting device which increases the voltage of a vehicle battery to the aimed high voltage. The lighting device is accommodated in a metal casing, and is set together with the lamp, or on the vehicle body near the lamp. Therefore, it is necessary that the lighting device is reduced in size and in weight as much as possible. In order to meet this requirement, the present inventor has conducted intensive research on the employment of a printed circuit board for the formation of a lighting circuit forming the lighting device except a starter circuit section using relatively bulky components; i.e., the formation of a DC-AC conversion circuit section adapted to convert the DC voltage of the vehicle battery into a high AC voltage.
On the other hand, the following facts have been found through a variety of experiments: In the case where the DC-AC conversion circuit section is formed on one printed circuit board, the output voltage is low in stability. The cause for this difficulty is as follows: The DC-AC conversion circuit section comprises: a power system for generating a high voltage which is applied to the starter circuit section; a control system for controlling various circuit sections forming the power system; and a power source system for supplying power to drive the power system and the control system. Therefore, if those systems are formed on one and the same printed circuit board, noises produced by the power system adversely affect the control system and the power source system, so that control signal noises and power source noises are produced. Those noises result in erroneous control operations, or they adversely affect the stability of the power source voltage, so that the lighting device becomes unstable in operation.
On the other hand, the control system is made up of analog circuits and digital circuits. Hence, noises due to the on-off operations of the digital circuits adversely affect the analog signals. Since the analog signals in those systems are employed as voltage and current detection signals in the power system, noises are mixed with the detection values of the detection signals; that is, the output voltage is varied by the noises.
The above-described difficulties may be eliminated by spacing the power system and t he control system away from each other, or by providing a shield wall between them. However, this method is not practical, because in order to space the power system and the control system away from each other or to provide the shield wall between them, it is necessary to increase the space as much which is to be occupied by those systems, which is an obstruction against the miniaturization of the lighting circuit. In addition, it is necessary to prevent the noises produced by the digital circuits from being applied to the analog circuits. To do so, similarly as in the above-described case, it is necessary to space those circuits away from each other, or to provide a shield wall between them. This fact will further make it difficult to miniaturize the light circuit.